This invention is concerned with the production of a braid structure which has a plurality of layers of stranded material which are laid down in the form of a hollow structure. Multi-layer braided material is made conventionally by forming a first layer of braid and then sequentially forming further layers over the original layer. Braids are currently produced using a "maypole" type of braider where an annular bedplate has a serpentine path formed by a pair of intersecting, serpentine tracks formed in it and movable package carriers adapted to travel along the tracks, each carrier carrying a package of stranded material which material forms the braid crossing the strands thereof from the various package carriers over each other at the intersections of the serpentine paths. The braid can be reinforced by having further "static" strands supplied from static carriers, i.e. carriers which are fixed in their location to the serpentine paths. Such a "static" strand is incorporated within the resultant braid structure to form a longitudinal strand. The braid is usually formed generally centrally of the bedplate and usually on its longitudinal axis.
To form a multi-layer braid, a series of maypole braiders may be set up with a common longitudinal axis and the braid as formed is passed serially through successive braiders so that separate layers are laid down one upon another. Alternatively, such a multilayer braid may be passed the requisite number of times through a single braider to build up the required number of layers. The braid thus formed can have any number of layers depending on the number of maypole braiders employed or the number of passes made as the case may be.
Multi-layer braid structures may be formed on a mandrel of a suitable shape, which mandrel can be removed subsequent to the formation of the structure, to result in the formation of a shaped, braid structure.
As an alternative to the maypole type of braider, it has been proposed to form a hollow braid in braiding machines of a tubular type such as is described, for example, in U.S. Pat. Nos. 4,621,560 and 4,753,150 assigned to Atlantic Research Corporation. In the machine disclosed in these patents the internal surface of a cylinder constituting the apparatus is composed of a plurality of ring members, each of a similar size which are axially arranged with respect to each other and each having a yarn carrier which can be moved axially of the cylinder. The ring members are adapted for rotation about the axis of the cylinder and the carriers are moved axially to interbraid the filaments. Stops are provided to limit and control the amount of axial movement. These machines, however, operate by discontinuous or integer motion of the members and carriers and in consequence are slow in operation. This in turn limits for practical purposes the uses to which such machines can be put and hence the nature and type of braided structures which can be produced.
A major problem of multilayer braids of the type described above is the tendency for the resultant braid to delaminate in service. Such braid structures are used in so-called "composites" formed, for example by impregnating such a braid structure with a resin material. While such a composite exhibits good mechanical properties in terms of tensile strength and tensile modulus in the plane of the layer, the mechanical properties of the composite transverse to the layer rely only on the shear strength of the matrix material and the bond strength between the matrix material and the fiber layer, since this is all that physically unites adjacent layers in the structure. Thus, when the composites are subjected to transverse loads there is a risk of inter-lamina failure between the layers of the braid.
Proposals have previously been made to overcome the de-lamination problem by introducing additional strands of material which extend transversly of the layers, during the braiding process. Some of these strands have been introduced randomly, whilst others have been introduced on a systematic radial basis by providing a mandrel which has apertures through which radial strands project. Such radial strands impart a degree of coherence between the braid layers, but their presence makes it difficult to interbraid the various strands from the package carriers and as a result the rate of formation of the braid slows down.
Another attempt to overcome the problems of delamination has been by introducing strands in a stitching operation which will unite and reinforce the layers of braid. This, although partially successful, does not give the level of strength or consistency which is required in many applications of such braid structures or of the composites formed therefrom.
In order to have an effective braid structure it is desirable for the various layers of the braid to be positively interlocked.